The Shambles,
your calendar does not count the days to the next millenium, it just counts the days till the start of the year 2000.
As there was no year zero,
the 1st millenium started in year 1 and ended with the year 1000,
the 2nd millenium started with year 1001 and ends with year 2000,
the 3rd millenium starts with year 2001 and ends with year 3000,
and so on...

Andreas